Identification and characterization of hydrostatic pressure sensors in marine brachyurans
Afferent activity, from the statocyst thread hairs of Carcinus maenas (L.), was altered by micro-pressure stimulation. Recordings showed that thread hair responses lag peak pressuring during low (1 kPa) pressure stimulation, but that response gain was maximal during such pressures. Increasing amplitude and decreasing cycle period reduced both lag and response gain. Data suggest a role for the thread hair in determining pressures and rates of pressure change of tidal periodicity. All available data were compatible with a transduction mechanism differentially activating paired bipolar neurones to a pressure increase. During angular acceleration evoked responses, post stimulus time histograms revealed an increased phase lag (re peak acceleration) with increasing pressure. Abrupt increases in directional class of unit responding to oscillation appeared at between 5-15 kPa. Comparative experiments, using the portunid crabs Necora puber (L.) and Liocarcinus depurator (L.) found that responses to rapid rates of pressure change were significant. No such responses were found in Carcinus maenas. Interneurone recordings were composed of abrupt elevations in spike frequencies followed by quiescent periods. In freshly caught animals, burst duration does not differ significantly throughout the tidal period. The quiescent period (interburst interval) was significantly longer at periods of expected low tide. Recordings made during pressure cycling show periodicities similar to that of the imposed pressure cycle. Applying 30 minute period, 30 kPa hydrostatic pressure cycles to loosely tethered crabs reduced burst duration significantly. Interburst interval increases significantly during pressure application. At cycle periods greater than 120 minutes, interburst intervals and burst durations were not significantly affected. At longer cycle periods (between 3-12 hrs) there were spike frequency and distributions of activity cycles comparable to those found in freshly caught, free-walking crabs. Preliminary results from recordings of statocyst afferents in the mysid Praunus flexuosus (Müller) show graded responses to micro-pressure stimulation.